Field of the Invention
The present invention relates to a fuel cell vehicle, and more particularly, to a method and apparatus for enhancing output efficiency of a fuel cell.
Discussion of the Related Art
A fuel cell system is a type of power generating system configured to convert chemical energy of fuel into electric energy in a fuel cell stack. However, when only a fuel cell is used as a power source of a vehicle, performance of the fuel cell is degraded since the fuel cell is in charge of all loads included in the vehicle and a sufficient amount of voltage required by a driving motor may not be supplied to degrade acceleration performance based on the output property whereby an output voltage is abruptly decreased in a high-speed driving region that requires a high voltage.
In addition, when a load is abruptly applied to a vehicle, an output voltage of a fuel cell is instantaneously and abruptly decreased and a sufficient amount of power may not be supplied to a driving motor to degrade vehicle performance (since electricity is generated based on a chemical reaction, an abrupt change in a load is beyond the fuel cell.) and a fuel cell may be unable to recover energy introduced from the driving motor during vehicle braking to degrade the efficiency of the vehicle system due to the unidirectional output property of the fuel cell.
Accordingly, a fuel cell hybrid vehicle has been developed in the related art. The fuel cell hybrid vehicle includes a high-voltage battery (or a super capacitor (super cap)) as an electricity accumulator as a separate power source configured to provide power required for motor driving as well as the fuel cell as a main power source. The fuel cell hybrid vehicle is driven by continuously outputting predetermined power from a fuel cell, and when power remains, a high-voltage battery is charged with the remaining power, and when power is insufficient (e.g., a low-output period of the fuel cell), the high-voltage battery complementarily outputs a voltage by as much as the insufficient amount of power to drive the vehicle.
The fuel cell hybrid vehicle may prevent use of the fuel cell in a period with low efficiency to enhance fuel efficiency via control for terminating an operation of the fuel cell in a low-output period and in particular, the fuel cell hybrid vehicle may be driven to stop power generation in the low-output period to enhance durability of the fuel cell hybrid vehicle. Particularly, the fuel cell hybrid vehicle may enhance fuel efficiency through a process of stopping and restarting power generation when necessary while vehicle is being driven (e.g., fuel cell stop/fuel cell restart procedures), that is, through an idle stop/release control procedure (e.g., an on/off control procedure for temporally stopping power generation of the fuel cell).
With regard to the method for embodying idle stop, various technologies have been developed, and in general, air supply of a fuel cell is stopped to prevent output generation. However, output is generated and present based on a reaction between hydrogen and the remaining air in the fuel cell when air supply is stopped. A fuel cell hybrid vehicle generally uses a method for charging a high-voltage battery using a main bus node to use output of a fuel cell and the main bus node is main power sources of many electronic loads, and thus the main bus node cannot have a low voltage equal to or less than a preset voltage. Accordingly, output of the fuel cell may be charged in the high-voltage battery up to a preset voltage region and output less than a preset voltage is consumed via self discharging. As a result, as output based on a reaction between hydrogen and remaining air in the fuel cell is self-discharged and a number of entrance times to idle stop increases, discharging output of the fuel cell accumulates to degrade fuel efficiency of the vehicle.